The iron law of prohibition states that when a substance (e.g. drugs, alcohol) becomes prohibited, it will begin to trade at more potent doses because it's cheaper to transport. For example, during 1930's prohibition, hard liquor became the drink of choice because you needed less of it for the same effect. But, by keeping it underground, it made it difficult to get a consistent, clean supply, just like with drug prohibition today, meaning quality would change as suppliers are routinely taken out or as they add other ingredients.